Answer:
CS(conditioned stimulus)
Explanation:
According to Pavlov, CS or conditioned stimulus is described as the stimulus that was previously neutral and triggers the response similar to an unconditioned stimulus due to repeated stimulation.
In the given example, the act of 'closing the door' has become a 'conditioned stimulus' as after repeatedly being associated with the naturally occurring stimulus, the students have learned to 'anticipate a pop quiz whenever Dr. Meyer closes the classroom door' which is the conditioned response. Thus, it justifies the above definition and hence, CS is the answer.
